The present invention relates to improvements in systems of the type commonly used to detect the movement of objects (e.g. intruders, pedestrians, vehicles, etc.) in a region under surveillance. More particularly, it relates to improvements in apparatus for supervising the operating status of such systems to assure that, in the event of a system failure, notice can be given to the system operator at the earliest possible time.
For many years now, microwave systems have been used to detect moving objects within a spatial region of interest. Operating on the well-known Doppler principle, such systems function to transmit microwave energy (e.g. X-Band radiation of about 10.5 GHz.) into a region under surveillance and to detect changes in frequency of the reflected energy, as produced by the Doppler effect. In conventional heterodyne microwave systems, separate and specially constructed diodes are respectively used to radiate and detect microwave energy. The radiating diode, usually a so-called Gunn diode, is disposed in a resonant cavity and responds to an applied DC voltage to produce microwave energy. An antenna, operatively coupled to the resonant cavity, serves to direct this microwave energy into the region of interest and to receive such energy upon being reflected by objects therein. The detecting diode, known as the receiver or mixing diode, functions to mix the transmitted and received energy to detect frequency shifts (i.e. the Doppler frequency) between the transmitted and received energy, such frequency shifts being characteristic of the particular type of target motion. The output of the receiver diode is amplified and processed, in a conventional manner, to eliminate spurious noise and false alarm-producing sources. The filtered signal may be used for any purpose, for example, to activate an alarm and/or to notify the police in an intruder detection application, or to open a door or change a pedestrian signal light in a pedestrian-detection application.
While microwave systems have enjoyed considerable commercial success in a variety of industries for detecting the presence of moving objects, continuous care must taken to assure that such systems are operating effectively at all times. For example, it is well known that the transmitter and receiver diodes of such systems are exceptionally susceptible to damage by relatively low-level electrostatic discharge (ESD), such as occurs during routine handling of such components during manufacture, installation and maintenance. Such ESD often causes a catastrophic failure of these diodes, in which case the diode either shorts-out or opens-up. Such a damaged diode is sometimes referred to as a "zapped" diode and, whether shorted-out or opened-up, the diode is rendered useless. A less severe discharge can produce a so-called "zinged" or "injured" diode, in which case the diode may still operate, but in a very noisy condition which has the effect of either substantially reducing the detection range or producing a continuous alarm condition (in an intruder detection application). Also, environmental changes, e.g., temperature changes, can cause the transmitter diode to either operate in a noisy condition or to stop oscillating all together, without any damage whatsoever to such diode.
Since microwave energy is invisible, a transmission failure, as occasioned, for example, by a zapped Gunn diode, cannot be detected by mere observation. The same is true, of course, of a failure in the signal processing circuitry of such systems, as may be produced by a failure of some other circuit element. While a non-operating (i.e. failed) microwave system is quickly discernible in applications where the system output is used on a frequent basis, e.g., to open doors, change pedestrian traffic lights. etc., the same is not true in intruder detection applications where the system is often required to operate, with high reliability, for long periods of time, sometimes weeks or months, without any visible or audible signs that the system is functional. Thus, to minimize the possibility that a failed microwave component will go undetected for such long time intervals, conventional microwave intruder detection systems commonly incorporate a so-called "supervisory" circuit which operates to continuously monitor the operating status of the transmitter and receiver elements of such systems and to activate a "trouble" alarm in the event of a failure of either of these elements. Such a "trouble" alarm commonly takes the form of a readily observable light-emitting diode or the like mounted on the microwave transceiver housing. Also, in the security business to provide increased protection against false alarms, it is becoming more common to back-up or supplement microwave systems with another intruder-detection scheme, such as passive-infrared or ultrasonic. In such "dual technology" systems, an "intruder" alarm is normally produced only in the event both detection schemes detect motion at substantially the same instant. See, for example, the microwave/passive-infrared system disclosed in the commonly assigned U.S. Pat. No. 4,660,024, to R. L. McMaster.
The above-noted McMaster patent discloses a heterodyne microwave system which includes supervisory circuitry for detecting either transmitter or receiver failure. In this system, the receiver diode is positioned directly within the energy field of a Gunn diode. In addition to providing a reference signal for subsequent Doppler frequency detection, the energy received directly from the Gunn diode serves to bias the receiver "on," thereby demonstrating to the supervisory circuit, which monitors the voltage level across the receiver diode, that the Gunn diode is indeed transmitting energy, and that the receiver diode is indeed receiving such energy. While this supervisory scheme is capable of detecting a catastrophic failure of either transmitter or receiver components, it does not function to supervise the operating status of the signal processing circuitry which operates on the output of the receiver diode. Moreover, it cannot detect a constant alarm condition, as can be produced by a "zinged" or noisy receiver diode.
In the commonly assigned U.S. application Ser. No. 479,050, entitled SUPERVISION OF AUTODYNE MICROWAVE MOTION-DETECTION SYSTEM, filed in the names of W. S. Dipoala and D. F. Pedtke on Feb. 12, 1990, there is disclosed an autodyne microwave motion-detection system in which a low-cost, general purpose diode is positioned in the resonant cavity of the microwave energy-generator to continuously supervise the operating status of such generator. In the event the energy induced on the supervisory diode drops below a predetermined threshold level, a trouble alarm is produced, indicating a transmission failure. Here again, there is no supervision of the system as a whole; rather, only the transmitting element is supervised. Also, like the McMaster supervisory scheme, no provision is made to detect a noisy transmitter diode or failure of the signal processing circuit.
In U.S. Pat. No. 4,833,450 to C. Buccola et al., there is disclosed a microwave/passive-infrared (i.e. dual-tech) intrusion detection system which incorporates a supervision circuit for detecting the operating status of the two detecting components. Such supervision circuit operates by counting the number of "false" trips of each component, i.e., a trip of one component not accompanied by a simultaneous trip by the other. When the number of false trips exceeds a certain preset value, a supervisory alarm is given, indicating that one or the other detecting components is faulty. While this supervisory scheme may have certain advantages, it is disadvantageous in that it is totally "passive" in nature. That is, to work, it must rely on the movement of an object or the occurrence of some event in the region of interest to produce the requisite false tripping of the alarm relays. Usually, such false trips will occur and be counted during periods in which the system is inactivated to allow authorized use of the protected premises. Should there be no opportunity for such events to occur, or should the events occur infrequently, as in the case, for example, where the protected region is a normally unoccupied warehouse, this supervision scheme would not work.